Example: Approximately 30% of the iv procainamide (PA) dose is metabolized to N-acetyl procainamide (NAPA) which is an active metabolite. Calculate the maximum plasma NAPA conc after administration of an iv dose of 800 mg PA if both PA and NAPA follow one-compartment PK model.
( k is 0.2 hr-1, Vd is 140 L, k(m) is 0.08hr-1 and Vd(m) is 100 L)

tmax (m)=

In k - In k(m)

k - k(m)
Cpmax(m)=

fm k D

Vd(m) (k(m)-k) (e-k t max(m) -e - k(m)tmax(m))

Answer

tmax(m) =

In 0.2 - In 0.08

0.2 hr-1 - 0.08 hr -1 = 7.6 hr
Cpmax(m) =

(0.3) 0.2 hr-1 800 mg

100L (0.08 - 0.2) hr-1 (e-0.2 hr-1 7.6 hr -e -0.08 hr-1 7.6 hr)
Cpmax(m) = 1.3 mg/L
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